Effect of micelle size and intermicellar distance on the chirality transfer in the intrinsic lyotropic cholesteric phases

Abstract

Lyotropic nematic and cholesteric liquid crystalline phases were produced using the racemic amphiphiles, potassium N-dodecanoyl-DL-alaninate (DL-KDDA) and DL-serinate (DL-KDDS) and their l-enantiomers, potassium N-dodecanoyl- L-alaninate (L-KDDA) and L-serinate (l-KDDS), respectively. The different racemic nematic and cholesteric phases were characterised by a polarising light microscope. In order to calculate the helical twisting powers (htp) of the cholesteric phases, their pitches were determined as a function of concentrations of the L-enantiomers. The results showed that the L-KDDA system had a higher helical twisting power than the L-KDDS system. Small-angle X-ray scattering (saxs) techniques were performed for determination of the structural differences between the racemic micelles and the chiral micelles of the racemic and intrinsic cholesteric phases, respectively. Saxs results showed that the intrinsic cholesteric micelles are more compact than their corresponding racemic micelles. These results were interpreted in terms of the different molecular arrangements of the chiral amphiphiles in micelles compared to the amphiphiles in racemic micelles. According to our experimental results and the results on induced cholesteric phases in the literature, we proposed that there must be (1) a crucial distance and (2) a critical micelle size in order to obtain chiral induction, supporting some theoretical studies reported in the literature. In addition, we also showed that chiral induction does not take place completely as was suggested by pairwise interaction or sterical interaction models in the literature, but through anisotropic chiral interactions via distorted micelles for which a crucial distance between the micelles and a critical micelle size are required.

Keywords:

• chiral induction
• racemic nematic phase
• cholesteric phase
• DL-racemic mixture
• L-enantiomer
• saxs
• laser diffraction
• pitch
• helical twisting power
• racemic micelle
• chiral micelle
• intermicellar interactions

Acknowledgements

We thank the Abant Izzet Baysal University Directorate of Research Projects Commission (BAP, grant no: 2012.03.03.529) and the staff in Prof. Dr. rer. nat. Frank Giesselmann's group/Physical Chemistry Department/University of Stuttgart/Germany.